Discussion on:

Office challenge: Why can't I use Excel's Subtotals feature?

The user is in edit mode. Therefore, the ribbon doesn't work.

The range she is trying to sort is formatted as a table. She will have to go to the Table Tools tab, Tools group, and Select "convert to range" to use subtotals. She cannot use both the subtotals and the table formatting.

The "blue" clue refers to the formatting that is applied by default when formatted as a table.

the Excel file may be shared, that disables quite a few functions, including Subtotals.

My be a formatted table or a ordinary sheet, by just typing the =subtotal(9, range) will calculate the values.

Adundon is right. The data has been formatted as a table.
You can filter and have a totals row at the bottom, but the Subtotal command is dimmed.

User needs to select a range (even if it's the entire sheet) or put the data in a table and select the table.

What is a table in Excel? I use them all of the time in Word, but haven't heard of them in Excel.

I removed the table "design" but was still left with what was recognized as a table in Excel. The solution = copy the data and paste special [values only] into another spreadsheet. Magically, the subtotal button is accessible and functionality available.